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Wednesday, August 1, 2012

Choosing Landing Sites Part 1: Opening new avenues

Previous missions' landing sites (NASA/JPL)

As I promised I will be concentrating on the details of
Curiosity’s landing site for the remainder of cruise time. As of this post the
space craft has entered approach phase and is now accessing autonomous
protocols in its onboard computer that will take it through the EDL phase
(entry, descent and landing) in the coming days.

The process of choosing landing sites is quite a long one
let alone describing the landing site itself. Therefore I’ll deal with this
topic by breaking down it into 3 posts to keep things short. In this part I’ll
talk about the process of choosing a landing site, its history and what factors
influence the decisions of scientists and engineers and policy makers involved
in this mission one way or another.

Careful selection of landing sites is essential to a
successful landing. NASA’s process of selection is quite rigorous and I believe
it is primarily because of this (besides the equally rigorous testing of space
craft components) that American missions have been so successful. I have
however no clue as to how Soviets did their game but in the end it all comes
down to what you actually know about your landing site choices. Information was
the name of the game.

There are 2 groups of factors that determine choice of
sites; scientific and engineering factors. The scientific reasons are quite
straightforward; what are the interesting features of the landing site (mineralogical,
geological, meteorological and so on)? Will these features help us (or not) to
answer questions related to the habitability of Mars and what makes this
landing site better than other choices? Habitability is the ability of an area
to support life whether past or present. MSL Curiosity is a mission designed
specifically to cater to the question of whether or not Mars can or could have
supported life and NOT to see if life exists today on Mars.

The engineering factors arise out of the need (to put it
bluntly) protect the rover. They tend to be fact checkers because despite the
interest generated by all the landing site choices in the scientific community
most of these choices have to be scrapped primarily because of engineering
constraints. So what are these constraints? They include dust factors (Mars’
ubiquitous red dust can interfere with all sorts of machine parts and
instruments, particularly optics which was a problem experienced in the last
rover missions Spirit and the still working Opportunity), atmospheric
conditions during landing, elevation of the Martian surface, surface roughness
and inclination and finally solar surface output. The last factor as well as
the first used to be a major concern for solar powered surface missions and are
responsible for the demise of the Spirit rover in the Martian winter of
2010/11. But this new gal is powered by radioactive decay of plutonium isotopes
and has no need for solar power which would have limited the landing site
choices to the tropics close to the equator. Surface roughness and inclination
is a major factor for missions especially those that use airbags to cushion
their landings like the 1997 Mars Pathfinder which featured Sojourner, the
first Martian rover and the 2004 Spirit and Opportunity rovers. Large rocks on
a high angle plane are a death wish for landers. Low plane angles and small
rocks here and there would make a safe zone. Low elevation is also important
because Mars’ atmosphere is a 100times thinner than ours. To ensure enough
Martian air is available to slow down the space craft during descent there
should be enough distance between the upper atmospheric boundary and the
surface. Temperatures on Mars can drop to as low as -80oC which can
damage electronics. This again restricts site choices to the tropics. Very few
landing sites exist that can satisfy all these criteria. What we need is a
better way of characterizing a landing site to open new avenues.

The mid 2000s introduced a major game changer in the art of
landing space hardware on Mars. Before mission planners had little quality
information to go by to determine whether a site was safe enough to land. The
Viking planners in 1976 were pretty sure about their first choices
until they had a better second look with the orbiters’ cameras they
brought along. They found rocky sites too dangerous to land machines that cost
I think the billions at the time. The landings were delayed but it gave the
planners time to choose better sites. Viking 1 and 2 both landed safely on
plains in the northern mid-latitudes of Mars (they were nuclear powered as well).
This example serves to illustrate the significance of having high quality data
on choice sites. The Viking orbiters had their cameras which helped map out the
entire planet at a higher resolution than ever before. In 2005 NASA’s Mar Reconnaissance
Orbiter (MRO) appeared at the scene sporting a the High Resolution ImagingScience Experiment (HiRISE), a telescopic camera with an incredible resolution
of 0.3m/pixel, powerful enough to resolve anything the size of a desk on Mars.

HiRISE being prepared for integration to MRO (Wikimedia commons)

This instrument enabled the acquisition of high resolution
images of all the landing site candidates (numbering over 60 sites at the beginning
of the selection process back in 2006). Together with other cutting edge MRO instruments
and with the help of other older orbiters like the 2001 Mars Odyssey orbiter,
the 1996 Mars Global Surveyor (stopped communicating in 2006) and Europe’s 2003
Mars Express, the MSL landing site selection work group had the most extensive
database of the red planet ever at their disposal! Surface roughness and dust factors could be better assessed and atmospheric phenomena could be probed with greater accuracy and with greater detail.

The MSL Curiosity mission has also a number of advantages
over the previous Mars surface missions. It has an advanced landing system (see
this post) with autonomous guidance capability. It has a power source
independent of the sun. Nuclear power generates a lot of excess heat which can
be piped to heat the rover during cold nights and winter. All these innovative
features combined with new information about landing site choices help to open
up new avenues to the surface. The advanced systems also meant the engineering
constraints could be relaxed a little. So the question of where the rover
should go was left mostly to the scientists to decide. Not an easy task
considering there were so many options. What to do? We’ll tackle that next
time.

If you would like to see detailed information on the selection process, click here.